You might want to give AnyData::Format::XML a look. From the introduction, it sounds like you should be able to use it to compare the contents of two XML files as if they were two databases, without having to slurp either file into memory. I imagine that you could then use database queries to find the differences between the two files and write those to another file, but then again, I've been doing a lot of database work recently, and you know what they say everything looks like to a man with a hammer. :)